1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system and, more particularly, to system having an arrangement suitable for enlargement zoom imaging.
2. Description of the Related Art
MRI systems for obtaining image data by MRI have been increasingly used for, e.g., medical diagnosis. Generally, in such an MRI system, an object to be examined is placed in a homogeneous static field, and an RF field, a slicing gradient field, a phase encoding gradient field, and a read-out gradient field are superposed on the homogeneous static field and are applied to the object in a predetermined sequence so as to excite magnetic resonance (MR) in the object. With this operation, MR signal data generated from a region of interest in the object are acquired, and the data of the region of interest are visualized to obtain diagnostic data.
In such an MRI system, in order to observe a region of interest, e.g., a morbid portion in detail, enlargement zoom imaging is sometimes performed to obtain an enlarged image formed by arbitrarily enlarging a region of interest. In a conventional enlargement zoom imaging technique, however, if a region of interest is deviated from the center of an imaging area (acquisition area of MR signal data) (i.e., if the central position of the region of interest does not coincide with that of the imaging area), the image of the region of interest extends outside an image plane (i.e., a display screen) as the magnification is increased. The image portion extending beyond the image plane does not satisfy the Nyquist condition, so that in many cases such an image portion becomes artifact of aliasing on the opposite side of the image plane. In such cases, therefore, it is difficult to increase the magnification.
Enlargement zoom imaging is generally performed by increasing the gradient strengths of phase encoding and read-out gradient fields when a data acquiring period is constant. However, if the gradient strengths are simply increased, the signal band of MR signals expands, and the Nyquist condition may not be satisfied. For example, if a region of interest of an object to be examined is deviated from the center of an imaging area, an artifact 72' due to aliasing is formed on the opposite side to an image 72 (to be obtained) of the object in an MR signal data acquisition area 71 on a Fourier plane, as shown in FIG. 1. In addition, even in a local excitation method, if a locally-excited region is deviated from the center of an image, the same problem as described above is posed when the resolution of an image is increased by increasing gradient strengths.
If an enlarged region of interest is located within the imaging region, what is required is only to prevent aliasing of the image portions other than the region of interest. In such a case, the above-noted problem can be easily solved by eliminating unnecessary band components by use of a filter. However, the elimination of unnecessary band components is not effective if the region of interest extends beyond the imaging region. In this case, the above-noted problem is solved in principle by increasing the sampling rate of MR signals in a fixed data collection time, mainly in the read direction. By so doing, the number of sampling points increases and the number of data points in the Fourier transformation increases, so that the Nyquist condition can be satisfied. As a result, it is possible to produce a high-resolution image which is substantially equivalent to that obtained by performing local zooming. However, since a very high sampling rate is used in the ultra high speed imaging method, it is difficult to further increase of the sampling rate of the high-bit A/D converter employed in the system, in light of the restrictions imposed by the hardware arrangement.
An artifact due to aliasing in the phase encoding direction can be prevented by decreasing the pitch of multistep encoding. With this operation, however, the number of encoding steps needed for obtaining the same imaging area is increased. Especially in an ultra high speed imaging method, since the acquisition time of MR signal data is limited by, e.g., countermeasures against the movement of an object to be examined and limitations associated with the transverse relaxation time (T*2), it is difficult to increase the number of encoding steps.